1. Field of the Invention
The present invention relates to an alkaline storage battery such as a sealed nickel-cadmium storage battery or the like, and more particularly to an alkaline storage battery having an improved current collector tab structure for increased high-rate discharge performance.
2. Prior Art
One conventional alkaline storage battery is shown in FIGS. 4 and 5 of the accompanying drawings. The known alkaline storage battery has an electrode assembly 24 comprising a strip-shaped positive electrode 22 and a strip-shaped negative electrode 33 which are wound as spiral coils with a separator 21 interposed therebetween. The electrode assembly 24 is housed in a nickel-plated steel case 25 which doubles as the negative terminal of the storage battery. The positive electrode 22 has a terminal edge 26 projecting from one end of the electrode assembly 24, whereas the negative electrode 23 has a terminal edge 27 projecting from the other end of the electrode assembly 24. The terminal edge 26 is connected to an upper current collector tab 28 which comprises a circular collecting tab 29 welded to the terminal edge 26 and a rectangular lead tab 30 extending from the circular collecting tab 29, the rectangular lead tab 30 having a width smaller than the diameter of the circular collecting tab 29. The lead tab 30 is spot-welded to a sealing cover 31 of the case 25, the sealing cover 31 serving as the positive terminal of the storage battery. The terminal edge 27 is connected to a lower current collector tab 32 which comprises a circular collecting tab 33 welded to the terminal edge 27 and a tongue-shaped lead tab 34 attached to the center of the circular collecting tab 33. The lead tab 34 is spot-welded to the inner surface of the bottom of the case 25 at the center of the circular collecting tab 33.
It is known that the high-rate discharge performance of the alkaline storage battery of such a construction greatly varies depending on the internal resistance of the storage battery.
The current collector tabs 28, 32 are spot-welded to the terminal edges 26, 27, respectively, of the electrode assembly 24. To spot-weld such a current collector tab to a terminal edge of an electrode assembly, one surface of the current collector tab is held against the terminal edge of the electrode assembly, and a pair of spot-welding electrodes is disposed on the other opposite surface of the current collector tab. During a spot-welding process, the welding current which is supplied between the spot-welding electrodes flows from the terminal edge of the electrode assembly to the electrode assembly itself through the current collector tab. In the case where the current collecting tab and lead tab of the current collector tab have the same thickness and are made of materials having the same resistivity, the overall thickness of the current collector tab may be increased or the current collector tab may have a plurality of current collecting tabs and lead tabs with a view to reducing the electric resistance of the entire current collector tab, as is well known in the art.
If the overall thickness of the current collector tab is increased in order to reduce the electric resistance thereof, then when the current collector tab is welded to the terminal edge of the electrode assembly, the loss current flowing through the current collector tab is increased, thereby reducing the welding current flowing through the joint between the terminal edge of the electrode assembly and the current collector tab. As a result, the joint cannot be welded effectively. It is therefore preferable to reduce the thickness of the current collector tab in order to minimize the loss current which flows during the welding process. If the current collector tab is thinned, however, the electric resistance of the lead tab on which the current is concentrated is increased.
If the current collector tab has a plurality of current collecting tabs and lead tabs are employed, then since they are successively spot-welded, the terminal edge of the electrode assembly tends to become brittle and the separator is liable to be damaged due to the heat applied in the welding process. Because of an increased number of welding steps required, the welding process is time-consuming and complex, resulting in a reduction in the rate of production. Consequently, it is impossible to reduce the electric resistance of the lead tab and hence the entire current collector tab.